Few people who saw it could forget the Late Late Show that was broadcast on October 22, 2004. To the left of Pat Kenny sat a shattered Rose Callaly, whose 30-year-old daughter Rachel had been murdered just 18 days before. In the chair beside her, his gaze slanting away occasionally towards the floor, his hands coolly clasped, was her son-in-law, Joe O’Reilly. As he spoke, Rose seemed to involuntarily recoil.

I was working as a reporter in the RTÉ Newsroom at the time, and – even though months would pass before O’Reilly was arrested and it would be another three years before he was convicted of his wife’s death – the strangeness of the situation was already apparent.

We hung around the TV building, eager for a glimpse at this man who was behaving so oddly in the aftermath of the murder of his wife; who had seemed to relish both the attention, and the opportunity to describe the scene he had found in the bedroom of the home they had shared in North Dublin.

There was something about O’Reilly, something not quite right.

What was that something? Was it evil? Does evil even exist? Why are we so fascinated by it?

These are the questions reporter Mick Peelo set out to answer in last night’s gripping Evil, the first in a two part Would You Believe? (RTÉ 1, Sunday, 10.35pm). “Can people be evil? Is evil a mental illness or a calculated choice that people make? Or are some people possessed by a force outside themselves?” Peelo asks.

We all know what we mean by evil but, given how often we invoke the term, it’s astonishing how little agreement there is on what it might actually constitute. “An act of pure evil” is the go-to headline for everything from school shootings to sexual abuse. And yet, as a philosophical construct, evil is seen as unfashionable, mired in quackery and religious superstition.

Peelo does a thorough job of exploring the various interpretations of it, calling on the views of priests, criminologists, neurologists, psychotherapists and folklorists. The only obvious omission from last night’s programme – the first of two – is the perspective of an actual psychopath.

The most compelling personal testimony comes, of course, from the Callalys, who are – unsurprisingly – unequivocal on the question of whether evil exists. “We were touched by evil when Rachel died,” Rose Callaly says firmly.

The couple describe how O’Reilly had invited them into his home a few weeks after Rachel’s murder and re-enacted what he said was his impression of how the murder might have unfolded. It was a process he repeated several times with the media, who dubbed it his “murder tour”.

“There was blood spatters on the wall, and on the ceiling. It was like being in an abattoir. He looked around at us then, and when he saw that we were so disturbed, his eyes started dancing in his head. His eyes were bouncing, and I said to myself ‘We’re with a madman’,” Jim Callaly says.

“That day, I came out of the house and I knew without a shadow of a doubt he had done it,” Rose says.

Rose Callaly has little doubt about whether people can be called evil. “I don’t honestly think you could do anything like that and have a conscience. I don’t honestly think he has a conscience. When you’d look into his eyes, there’s nothing there,” she said.

Jim Callaly adds: “I think there was evil in him, and I think certain things bring out the evil. Greed, jealousy, hatred. He arranged for Rose to find her body that day. That was pure evil. That’s what they get their kick out of.

“It’s as if he’s just a tin box with nothing in it.”

Psychotherapist Christine Louise De Canonville – who also has personal experience of psychopathic violence – offers a somewhat black and white view on the genesis of evil: she sees it as rooted in the extreme form of self-regard we know as narcissism.

“Regular” narcissists are manipulative, lack empathy and betray signs of an extreme form of self-love. Malignant narcissists are “very violent, very sadistic” and take pleasure in the harm they exert on others. Then there are psychopaths who are, she says, “sadistic, anti-social and paranoid”.

Other experts are called in to give their view on whether evil is part of the human condition – or something else. Dr Clare Kelly, assistant professor of functional neuroimaging, is clear that violence and lack of empathy, like all human behaviour, comes from the brain. Brain imaging on psychopathic individuals shows lower levels of activity in the amygdala, the part of the brain that controls our empathy response. But Kelly stops short of describing it as “the evil spot”.

Ultimately, however we are wired, every behaviour is rooted in personal responsibility – something borne out by the fact that not all people with psychopathic-type brain responses are prone to violence.

Professor of Criminology at University College Cork, Colin Sumner, suggests that the notion of a criminal mind has been largely discredited, in favour of a more nuanced understanding. For Sumner, “evil” is not an absolute, but is determined by the level of moral outrage it causes. “Even with the most serious actions, like killing people, in some circumstances, we wouldn’t be morally outraged.”

It also comes back to personal responsibility for psychiatrist Ivor Browne. The concept of evil, he suggests, can a dangerous notion when the desire to eradicate it leads to equally dark extremes of human behaviour.

The supernatural dimension of evil is the final one explored in the documentary. Fr Pat Collins, who conducts exorcisms, believes it is “utterly naïve” to see it as a simply a human phenomenon.

De Canonville is likewise open to the notion of a spiritual explanation, describing how, when the brother who once tried to slice the top of her head off with a carving knife went into his psychopathic rages, his face would change. “His features would change. His eyes would penetrate you. He wasn’t there anymore. My brother looked demonic. He looked as if he was possessed.”

Ultimately, Peelo declines to declare for other side, suggesting instead that we might perhaps need evil to keep societies in check. It is a somewhat unsatisfactory conclusion to an otherwise bleakly fascinating documentary – but then perhaps that is as it should be. Human nature, in all its potential for good and darkness – evil, even – defies neat explanations.

For the Callalys, stoic but still obviously pained some 13 years on, there is no end, no closure, no relief, only a single, unavoidable conclusion.

“I came across the murderer, Joe O’Reilly,” Jim Callaly says, slipping into the past tense in perhaps the clearest signal of all about his feelings about his son-in-law. “He was pure evil.”

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